Magnetic garnet single crystal film formation substrate, optical element and production method of the same

ABSTRACT

A magnetic garnet single crystal film formation substrate  2  for growing a thick magnetic garnet single crystal film, wherein crystal defects, warps, cracks and flaking, etc. are not caused, by liquid phase epitaxial growth is provided. The substrate  2  comprises a base substrate  10  composed of a garnet-based single crystal being unstable with a flux used for the liquid phase epitaxial growth; a buffer layer  11   a  composed of a garnet-based single crystal thin film formed on a crystal growing surface  10   a  of said base substrate  10  and being stable with said flux; and a protective layer  11   b  formed at least on side surfaces  10   b  of said base substrate  10  crossing with said crystal growing surface of said base substrate  10  and being stable with said flux. By using the substrate, a high quality magnetic garnet single crystal film can be produced. The magnetic garnet single crystal film is used as an optical element, such as a Faraday element, used for an optical isolator, optical circulator and magneto-optical sensor, etc.

TECHNICAL FIELD

The present invention relates to a magnetic garnet single crystal filmformation substrate for growing a magnetic garnet single crystal filmof, for example, bismuth-substituted rare earth iron garnet (Bi-RIG) byliquid phase epitaxial growth, a method of producing a single crystalfilm for performing crystal growth by using the substrate, and a singlecrystal film and an optical element produced by the production method.

BACKGOUND ART

As a material of an optical element, such as a Faraday rotator used inan optical isolator, optical circulator and optical magnetic fieldsensor, etc., what obtained by growing a magnetic garnet single crystalfilm on a single crystal substrate by epitaxial growth is generallyused. The magnetic garnet single crystal film to be grown on thesubstrate is desired to have a large Faraday rotation coefficient toobtain desired Faraday effects. Also, to form a high quality singlecrystal film by epitaxial growth, it is required that a lattice constantdifference between the substrate single crystal and the single crystalto be grown is as small as possible in a temperature range from a filmforming temperature to the room temperature.

It is known that the Faraday rotation coefficient of the magnetic garnetsingle crystal film remarkably increases by substituting a part of therare earth components with bismuth. An increase of a bismuthsubstitution amount brings an increase of a lattice constant of themagnetic garnet single crystal film at the same time, so that asubstrate material used for the film forming is also required to have alarger lattice constant and, for example, gadolinium gallium garnet(GGG) added with Ca, Zr and Mg, etc. to obtain a large lattice constantis used as the single crystal substrate material (the patent article 1:The Japanese Examined Patent Publication No. 60-4583).

However, in the case of growing bismuth-substituted rare earth irongarnet single crystal on the GGG single crystal substrate added with Ca,Zr and Mg, etc. to be a thick film (a film thickness of, for example,200 μm or more), the substrate and the single crystal layer during andafter the film forming are liable to warp and crack, which is a cause ofdeclining production yields at the time of the film forming andprocessing.

To eliminate the problem, the present inventors have proposed a garnetsingle crystal substrate of a specific composition wherein a thermalexpansion coefficient within a face being perpendicular to the crystalorientation <111> is a value extremely close to that of thebismuth-substituted rare earth iron garnet in a temperature range fromthe room temperature to 850° C. (the patent article 2: The JapaneseUnexamined Patent Publication No. 10-139596). By using this singlecrystal substrate, it is possible to form a thick filmbismuth-substituted rare earth iron garnet single crystal film by liquidphase epitaxial growth wherein crystal defects, warps and cracks, etc.are not caused.

However, it was found by the present inventors that the garnet singlecrystal substrate of this specific composition was unstable with leadoxide flux used as a deposition medium at the time of performing theliquid phase epitaxial growth of bismuth-substituted rare metal irongarnet (Bi-RIG) single crystal film, so the yields of obtaining highquality bismuth-substituted rare earth iron garnet single crystal waspoor. Particularly, it was found that this tendency was strong in asubstrate composition containing Nb or Ta.

Thus, the present inventors have developed a substrate, wherein a bufferlayer made by a garnet based single crystal thin film being stable witha flux is formed on a bottom surface of a base substrate made by agarnet based single crystal being unstable with a flux, and filedpreviously (refer to the patent article 3: PCT/JP02/06223).

However, when the buffer layer was formed only on the bottom surface asa growing surface of the base substrate, side surfaces of the basesubstrate reacted with a flux and flaked away, which turned out to be aproblem of deteriorating a quality of the magnetic garnet single crystalfilm and declining a production yield.

The present invention was made in consideration of the abovecircumstances and has an object thereof to provide a magnetic garnetsingle crystal film formation substrate capable of stably forming athick magnetic garnet single crystal film, wherein crystal defects,warps, cracks and flaking, etc. are not caused, having a high quality ata high yield by liquid phase epitaxial growth, and an optical elementand the production method.

DISCLOSURE OF THE INVENTION

To attain the above object, there is provided a magnetic garnet singlecrystal film formation substrate for growing a magnetic garnet singlecrystal film by liquid phase epitaxial growth, comprising:

a base substrate composed of a garnet-based single crystal which isunstable with a flux used for the liquid phase epitaxial growth;

a buffer layer composed of a garnet-based single crystal thin filmformed on a crystal growing surface of the base substrate and beingstable with the flux; and

a protective layer formed at least on side surfaces of the basesubstrate crossing with the crystal growing surface of the basesubstrate and being stable with the flux.

The above flux is not particularly limited, but is a flux including, forexample, a lead oxide and/or a bismuth oxide. Note that “being unstablewith a flux” in the present invention means that, in a so-calledsupersaturated state where a solute component in the flux starts tocrystallize by using an object substance (a base substrate or bufferlayer) as a core, at least a part of a material composing the objectsubstance is eluted to the flux and/or at least a part of the fluxcomponent is diffused to the object substance to hinder the liquid phaseepitaxial growth of a single crystal film. Also, “being stable with aflux” means a reverse phenomenon of the “being unstable with a flux”.

According to the present invention, it is possible to select a garnetsingle crystal substrate of a specific composition having an extremelyclose thermal expansion coefficient to that of a magnetic garnet singlecrystal, for example, bismuth-substituted rare earth iron garnet to bean object of forming by liquid phase epitaxial growth and, even when thesubstrate is unstable with the flux, stable liquid phase epitaxialgrowth can be performed. It is because a buffer layer being stable witha flux is formed on the crystal growing surface of the base substrate.

Particularly, in the present invention, since a protective layer otherthan the buffer layer is formed also on the side surfaces of the basesubstrate crossing with the crystal growing surface of the basesubstrate, the side surfaces of the base substrate do not react with aflux, so that the quality of the magnetic garnet single crystal filmimproves and the production yield also improves.

Thus, in the present invention, a bismuth-substituted rare earth irongarnet single crystal film used in a Faraday rotator and other opticalelements can be formed by liquid phase epitaxial growth at a highquality at a high production yield while suppressing arising of crystaldefects, warps, cracks and flaking, etc. Namely, according to thepresent invention, a relatively thick (for example, 200 μm or more) andwide (for example, a diameter of 3 inches or more) magnetic garnetsingle crystal film can be obtained by liquid phase epitaxial growth.

Preferably, the protective layer is composed of the same film as thebuffer layer. By composing the protective layer by the same film as thebuffer layer, the protective layer can be formed at the same time asforming the buffer layer, and the production procedure becomes easy.

Note that in the present invention, it is not necessary to form amagnetic garnet single crystal film on the protective layer in apositive way, so that the protective layer may be any as far as it is afilm being stable with a flux and does not have to be a garnet basedsingle crystal thin film. Accordingly, the protective layer may becomposed of a silicon oxide film or an aluminum oxide film, etc. Theprotective layer may be formed by a thin film formation method, such asthe chemical solution deposition method, the sputtering method, theMOCVD method and the pulse laser deposition method, etc. separately fromthe buffer layer.

Preferably, the base substrate has approximately the same thermalexpansion coefficient as that of the magnetic garnet single crystalfilm. For example in a temperature range of 0° C. to 1000° C., thedifference of the thermal expansion coefficient of the base substrate iswithin a range of ±2×10⁻⁶/° C. or less with respect to the thermalexpansion coefficient of the magnetic garnet single crystal film.

By making the thermal expansion coefficient of the base substrateapproximately the same with that of the magnetic garnet single crystalfilm, deterioration of quality, such that a film after epitaxial growthcomes off from the substrate and cracks and chips, etc. (hereinafter,also referred to as “cracks, etc.”), can be effectively prevented. It isbecause, at the time of forming a magnetic garnet single crystal film byepitaxial growth, the temperature rises nearly 1000° C. and returns tothe room temperature, so cracks, etc. easily arise on the epitaxialgrowth film when the thermal expansion coefficients are different.

Note that the thermal expansion coefficient of the buffer layer is notnecessarily approximately the same with that of the magnetic garnetsingle crystal film. It is because a film thickness of the buffer layeris extremely thin with respect to a thickness of the base substrate andthe thermal expansion difference affects a little on the epitaxialgrowth film.

Preferably, the base substrate has approximately the same latticeconstant as that of the magnetic garnet single crystal film. Forexample, the difference of the lattice constant of the base substrate isin a range of ±0.02 Å or less with respect to the lattice constant ofthe magnetic garnet single crystal film.

By making the lattice constant of the base substrate approximately thesame as that of the magnetic garnet single crystal film, the magneticgarnet single crystal film is easily grown by liquid phase epitaxialgrowth.

Preferably, the base substrate includes Nb or Ta. When Nb or Ta isincluded in the base substrate, the thermal expansion coefficient and/orlattice constant of the base substrate is easily made approximately thesame as the lattice constant of the magnetic garnet single crystal film.Note that when Nb or Ta is included in the base substrate, stabilitywith a flux is liable to decline.

Preferably, the buffer layer is a garnet-based single crystal thin filmsubstantially not including Nb and Ta. It is because a garnet-basedsingle crystal thin film substantially not including Nb and Ta isrelatively stable with a flux.

Preferably, the buffer layer is

expressed by a general formula R₃M₅O₁₂ (note that R is at least one ofrare earth elements and M is one selected from Ga and Fe) or

an X-substituted gadolinium gallium garnet (note that X is at least oneof Ca, Mg and Zr).

The buffer layer composed of a material as above is preferable for beingrelatively stable with a flux and, moreover, having a lattice constantclose to that of a magnetic garnet single crystal film.

Preferably, a thickness of the buffer layer is 1 to 10000 nm, morepreferably 5 to 50 nm, and a thickness of the base substrate is 0.1 to 5mm, more preferably 0.2 to 2.0 mm. When the thickness of the bufferlayer is too thin, an effect of the present invention becomes small,while when it is too thick, the cost becomes high and it is liable toadversely affect on the epitaxial growth film, such as cracks, due to adifference of thermal expansion coefficient. Also, when the thickness ofthe base substrate is too thin, it is liable that mechanical strengthbecomes insufficient and handling becomes deteriorated, while when it istoo thick, arising of cracks, etc. is liable to increase.

A method of producing a magnetic garnet single crystal film according tothe present invention includes the step of growing a magnetic garnetsingle crystal film on the buffer layer by using the magnetic garnetsingle crystal film formation substrate of the present invention by aliquid phase epitaxial growth method.

A method of producing an optical element according to the presentinvention includes the steps of forming a magnetic garnet single crystalfilm by using the method of producing a magnetic garnet single crystalfilm of the present invention, and after that, removing the basesubstrate and buffer layer in order to form an optical element composedof the magnetic garnet single crystal film.

Preferably, by removing the base substrate and the buffer layer andremoving a magnetic garnet film formed on side surfaces of the basesubstrate, and

leaving only a magnetic garnet single crystal film formed on a crystalgrowing surface of the base substrate,

an optical element composed of the magnetic garnet single crystal filmis formed.

In the present invention, a magnetic garnet film is formed also on sidesurfaces of the base substrate. In the present invention, the magneticgarnet film formed on the side surfaces of the base substrate has apoorer quality comparing with the magnetic garnet single crystal filmformed on a crystal growth surface of the base substrate, so that thispart is preferably removed for using as an optical element.

An optical element according to the present invention is obtained by themethod of producing an optical element of the present invention.

BRIEF DESCRIPTION OF DRAWINGS

Below, the present invention will be explained based on embodimentsshown in drawings.

FIG. 1 is a sectional view of a magnetic garnet single crystal filmformation substrate according to an embodiment of the present inventionand a bismuth-substituted rare earth iron garnet single crystal filmgrown by using the same;

FIG. 2 is a schematic view of an apparatus for growing crystal;

FIG. 3A is an optical microscope picture of a surface when growingcrystal by using a magnetic garnet single crystal film formationsubstrate according to an example of the present invention;

FIG. 3B is an optical microscope picture of a surface when growingcrystal by using a magnetic garnet single crystal film formationsubstrate according to a comparative example of the present invention;

FIG. 4A is a SEM image of a surface of a magnetic garnet single crystalfilm formation substrate according to an example of the presentinvention;

FIG. 4B is a SEM image of a section of the substrate shown in FIG. 4A;

FIG. 5 is a sectional SEM image in a state that a bismuth-substitutedrare earth iron garnet single crystal film is formed on a surface of amagnetic garnet single crystal film formation substrate according to anexample of the present invention;

FIG. 6A is a SEM image of a surface in a state that abismuth-substituted rare earth iron garnet single crystal film is formedon a surface of a magnetic garnet single crystal film formationsubstrate according to a comparative example 1 of the present invention;and

FIG. 6B is a SEM image of a surface in a state that abismuth-substituted rare earth iron garnet single crystal film is formedon a surface of a magnetic garnet single crystal film formationsubstrate according to a comparative example 2 of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

As shown in FIG. 1, a magnetic garnet single crystal film formationsubstrate 2 in the present embodiment comprises a base substrate 10 anda buffer layer 11 formed continuously over a bottom surface (crystalgrowing surface) 10 a and entire side surfaces 10 b of the basesubstrate 10. The base substrate 10 has a lattice constant and thermalexpansion coefficient being extremely close to those of a magneticgarnet single crystal film 12 made by bismuth-substituted rare earthiron garnet single crystal but is unstable with a lead oxide flux. Thebuffer layer 11 is composed of a garnet-based single crystal thin filmbeing stable with a lead oxide flux. In the present embodiment, thebuffer layer 11 is composed of a bottom surface buffer layer 11 a formedon the bottom surface 10 a of the base substrate 10 and a side surfacebuffer layer (protective layer) 11 b formed on the side surfaces 10 b ofthe base substrate 10.

A bismuth-substituted rare earth iron garnet single crystal film 12 isgrown on the buffer layer 11 in the substrate 2 by liquid phaseepitaxial growth. The base substrate 10 has preferable lattice matchingproperty with the single crystal film 12 for growing the magnetic garnetsingle crystal film 12 via the buffer layer 11 and the linear thermalexpansion coefficient of the substrate has a characteristic close tothat of the single crystal film 12.

The base substrate 10 is composed of, for example, nonmagneticgarnet-based single crystal expressed by a general formulaM1_(x)M2_(y)M3_(z)O₁₂. In this general formula, M1 is an element, forexample, selected from Ca, Sr, Cd and Mn. As the M1, those existingstably when having 2+ valence number, having a coordination number of 8,having an ion radius in a range of 0.096 to 0.126 nm in this state arepreferable. The M2 is an element, for example, selected from Nb, Ta andSb. As the M2, those existing stably when having 5+ valence number,having a coordination number of 6, having an ion radius in a range of0.060 to 0.064 nm in this state are preferable. The M3 is an element,for example, selected from Ga, Al, Fe, Ge, Si and V. As the M3, thoseexisting stably when having 3+, 4+ or 5+ valence number, having acoordination number of 4, having an ion radius in a range of 0.026 to0.049 nm in this state are preferable. Note that the ion radius is avalue of an effective ion radius defined by R. D. Shannon. The M1, M2and M3 may be a single element or combination of two or more kinds ofelements.

Furthermore, in the element of the M1, a part thereof may be substitutedwith an element M4 which is substitutable with Ca or Sr in thecomposition in a range of less than 50 atomic % in accordance with needin order to adjust the valence number and lattice constant. As the M4,for example, at least one kind selected from Cd, Mn, K, Na, Li, Pb, Ba,Mg, Fe, Co, rare earth elements and Bi and those capable of having acoordination number of 8 are preferable.

Also, in the M2, in the same way as in the M1, a part thereof may besubstituted with an element M5 which is substitutable with Nb, Ta or Sbin the composition in a range of less than 50 atomic %. As the M5, forexample, at least one kind selected from Zn, Mg, Mn, Ni, Cu, Cr, Co, Ga,Fe, Al, V, Sc, In, Ti, Zr, Si and Sn and those capable of having acoordination number of 6 may be preferably mentioned.

The single crystal substrate having a composition as above has a thermalexpansion coefficient close to that of bismuth-substituted rare earthiron garnet single crystal to be grown and has preferable latticematching property with the single crystal. Particularly, in the abovegeneral formula, those having values of “x” in a range of 2.98 to 3.02,“y” in a range of 1.67 to 1.72, and “z” in a range of 3.15 to 3.21 arepreferable.

The thermal expansion coefficient of the base substrate 10 having theabove composition is 1.02×10⁻⁵/° C. to 1.07×10⁻⁵/° C. or so in the roomtemperature to 850° C., which very much approximates the linear thermalexpansion coefficient of 1.09×10⁻⁵/° C. to 1.16×10⁻⁵/° C. ofbismuth-substituted rare earth iron garnet single crystal film in thesame temperature range.

Also, the thickness of the base substrate 10 is not particularlylimited, but the thickness is preferably 1.5 mm or less in terms ofsuppressing arising of cracks and warps, etc. of the substrate and thesingle crystal film at the time of film forming when forming abismuth-substituted rare earth iron garnet single crystal film having afilm thickness of 200 μm or more, so that a preferable single crystalfilm can be obtained. When the thickness of the base substrate exceeds1.5 mm, there is a tendency that arising of cracks increases near aboundary surface of the substrate and the single crystal film along withan increase of the thickness. Also, when the thickness of the singlecrystal substrate 10 becomes too thin, mechanical strength becomes smalland handleability becomes poor, so that those having a thickness of 0.1mm or more are preferable.

The buffer layer 11 formed on the single crystal substrate 10 iscomposed of a garnet-based single crystal thin film. As the garnet-basedsingle crystal thin film, what expressed by a general formula R₃M₅O₁₂(note that R is at least one kind of rare earth elements and M is onekind selected from Ga and Fe), or X-substituted gadolinium galliumgarnet (note that X is at least one kind of Ca, Mg and Zr) may bementioned.

Among these, it is preferable to use one kind selected from neodymiumgallium garnet, samarium gallium garnet, gadolinium gallium garnet andX-substituted gadolinium gallium garnet (note that X is at least onekind of Ca, Mg and Zr), but it is not limited to the above as far as itis a garnet-based material being stable with a lead oxide flux.

A method of producing the base substrate 10 in the magnetic garnetsingle crystal film formation substrate of the present invention is notparticularly limited, and a method commonly used in producing aconventional GGG single crystal substrate, etc. can be applied.

For example, first, a homogenous molten mixture is prepared wherein eachof or two or more selected kinds of elements expressed by M1, elementsexpressed by M2 and elements expressed by M3 in the above generalformula and each of or two or more selected kinds of elements expressedby M4 and elements expressed by M5 used depending on the case arecontained at a predetermined ratio. Next, a polycrystalline body isformed from the molten mixture, for example, by dipping GGG seedcrystal, etc. having a long axis direction of <111> perpendicular to theliquid surface and pulling up while slowly rotating.

Since there are a large number of cracks on the polycrystalline body, asingle crystal portion without a crack is selected from that and, afterconfirming the crystal orientation, it is dipped as a seed crystal inthe above molten mixture so that the crystal orientation <111> becomesperpendicular to the liquid surface and pulled up while slowly rotating,consequently, a single crystal without a crack is formed. Next, thesingle crystal is cut perpendicular to the growth direction to be apredetermined thickness, performing mirror polishing on the bothsurfaces, and performing etching processing, for example, with heatphosphoric acid, etc. to obtain the base substrate 10.

The buffer layer 11 composed of a garnet-based single crystal thin filmhaving the above composition is formed on the thus obtained basesubstrate 10 by a sputtering method, a CVD method, a pulse laserdeposition method, a chemical solution deposition method or other thinfilm formation technique.

In the present embodiment, the buffer layer 11 is formed as a protectivelayer not only on the bottom surface 10 a of the base substrate 10 buton the side surfaces 10 b of the base substrate 10. Therefore, in thepresent embodiment, not only the bottom surface 10 a but the sidesurfaces 10 b of the base substrate 10 are polished and the films areformed under a condition that the buffer layer 11 is formed also on theside surfaces 10 b. For example, when applying a sputtering method, bymaking the film formation pressure 0.1 to 10 Pa, and preferably 1 to 3Pa, the buffer layer (protective layer) 11 b is formed also on the sidesurfaces 10 b in addition to the bottom surface 10 a of the basesubstrate 10.

Alternately, when applying the MOCVD method, the buffer layer 11 isformed also on the side surfaces 10 b in addition to the bottom surface10 a of the base substrate 10 under a general film formation condition.Also, when applying a chemical solution deposition method, such as a solgel method, it is sufficient to immerse the entire surface of the basesubstrate 10 in a solution for forming the buffer layer 11. Alternately,a solution for forming the buffer layer 11 may be applied by using abrush or spray, etc.

In the present invention, a material of the bottom surface buffer layer11 a formed on the bottom surface 10 a of the base substrate 10 and amaterial of a side surface buffer layer 11 b formed on the side surfaces10 b of the base substrate 10 may be different or same. Note that thebuffer layers 11 a and 11 b are preferably formed at a time. By doingso, a step of forming the buffer layer 11 can be reduced.

Note that the side surface buffer layer (side surface protective layer)11 b may be poor in the film quality comparing with the bottom surfacebuffer layer 11 a. It is because the garnet film 12 b to be formed onthe surface of the side surface buffer layer 11 b is a part that may beremoved in a later step. Also, a thickness of the bottom surface bufferlayer 11 a and a thickness of the side surface buffer layer 11 b may besame or different. Note that the thicknesses of the buffer layers 11 aand 11 b are preferably in a range of 1 to 10000 nm, and more preferably5 to 50 nm. When the thickness of the buffer layers 11 a and 11 b aretoo thin, an effect of the present invention becomes small, while whentoo thick, the cost increases and cracks and other adverse effects tendto be given to an epitaxial growing film due to a difference of thethermal expansion coefficient, etc.

By using a magnetic garnet single crystal film formation substrate 2configured as above, a magnetic garnet single crystal film 12 composedof a bismuth-substituted rare earth iron garnet single crystal film isformed by the liquid phase epitaxial growth method. In the presentembodiment, the magnetic garnet single crystal film 12 is formed notonly on the bottom surface but also on the side surfaces of the magneticgarnet single crystal film formation substrate 2. Note that the sidesurface garnet film 12 b formed on the side surfaces of the substrate 2is generally poor in the film quality comparing with the bottom surfacesingle crystal film 12 a formed on the bottom surface and removed lateron.

A composition of the bismuth-substituted rare earth iron garnet singlecrystal film composing magnetic garnet single crystal film 12 isexpressed by, for example, a general formula Bi_(m)R_(3-m)Fe_(5-n)MnO₁₂(R is at least one kind of rare earth elements, M is at least one kindof element selected from Ga, Al, In, Sc, Si, Ti, Ge and Mg, and rangesof “m” and “n” are 0<m<3.0 and 0≦n≦1.5 in the formula).

In the general formula, as the rare earth element expressed by R, forexample, Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, etc. arementioned, and one or more kinds of these may be included.

In the single crystal, a part of the rare earth element expressed by Ris substituted with bismuth, the ratio of the substitution with bismuthis expressed by “m” and the value of “m” is in a range of 0<m<3.0.Particularly when “m” is in the range of 0.5 to 1.5, the thermalexpansion coefficient of the single crystal and that of the singlecrystal substrate become extremely close, so it is advantageous. Also,“M” is a nonmagnetic element substitutable with Fe, which is Ga, Al, In,Sc, Si, Ti, Ge and Mg, and one or more kinds of these may be included.The ratio “n” of the substitution with Fe in the nonmagnetic element isselected from the range of 0 to 1.5.

To form a bismuth-substituted rare earth iron garnet single crystal filmby the liquid phase epitaxial growth method, a homogenous molten mixturecontaining a predetermined ratio of (1) a bismuth oxide, (2) at leastone kind of rare earth element oxide, (3) an ion oxide and if necessary(4) an oxide of at least one kind of element selected from Ga, Al, In,Sc, Si, Ti, Ge and Mg used depending on the case is prepared. As asolute for precipitation, a lead oxide is normally used as a maincomponent, but a bismuth oxide or other solute for precipitation may beused. Also, a boron oxide, etc. may be also included as a crystal growthauxiliary if desired.

Next, by dipping the substrate 2 of the present invention in the moltenmixture (a solute flux 22 in a crucible 20 shown in FIG. 2), a singlecrystal is grown by epitaxial-growth from the molten mixture on asurface of the buffer layer 11 of the substrate 2 so as to form amagnetic garnet single crystal film 12. A temperature of the moltenmixture at this time varies depending on a composition of the materialmixture, but is normally selected from a range of 600 to 1000° C. Also,the substrate 2 may be subjected to epitaxial growth by being left inthe molten mixture or by being suitably rotated by a rotation axis 24shown in FIG. 2. When rotating the substrate 2, the speed of therotation is preferably 10 to 200 rpm. Also, the speed of film formationis normally 0.08 to 0.8 μm/minute or so. The dipping time variesdepending on the film formation speed and a desired film thickness, etc.and cannot be determined as a rule, but is normally 10 to 100 hours orso.

After finishing the epitaxial growth, the substrate 2 is pulled out fromthe molten mixture and adhered molten mixture is sufficiently swishedoff, then, cooled to the room temperature. Next, after a caked substanceof the molten mixture adhered to the surface of the formed singlecrystal film is removed by being dipped in a mineral acid solution ofaqua fortis, etc., it is washed with water and dried. The thickness ofthe magnetic garnet single crystal film 12 composed ofbismuth-substituted rare earth iron garnet single crystal formed on thesubstrate 2 as above is normally in a range of 100 to 1000 μm. Also, thethermal expansion coefficient is 1.0×10⁻⁵/° C. to 1.2×10⁻⁵/° C. or so inthe room temperature to 850° C.

As explained above, the crystal structure and composition of thebismuth-substituted rare earth iron garnet single crystal film formed onthe substrate 2 can be identified respectively by X-ray diffractionpattern and by X-ray fluorescent analysis, etc. Also, performance of thesingle crystal film 12 can be evaluated by removing the substrate 2 (thebase substrate 10+the buffer layer 11) from the single crystal film 12by polish processing, etc., then, performing polish processing on bothsurfaces of the film 12, providing non-reflection film on the bothsurfaces, and obtaining a Faraday rotation coefficient, transmissionloss and temperature property, etc.

In the present embodiment, when using the single crystal film 12 as anoptical element, it is preferable that the garnet film 12 b formed onthe side surfaces of the substrate 2 is removed by polishing processing,etc. to form the optical element only by the single crystal film 12 aformed on the bottom surface of the substrate 2.

Note that the present invention is not limited to the above embodimentand may be variously modified within the scope of the present invention.For example, in the embodiment shown in FIG. 1, the buffer layer 11 wasformed only on the bottom surface 10 a and the side surfaces 10 b of thebase substrate 10, but the buffer layer 11 may be formed allover thebase substrate 10 including the upper surface 10 c of the base substrate10.

Also, in the present invention, the protective layer (side surfacebuffer layer 11 b) formed on the side surfaces 10 b of the basesubstrate 10 does not have to be the same as the bottom surface bufferlayer 11 a and does not have to be a garnet based single crystal thinfilm as far as it is a film being stable with a flux. Accordingly, theprotective layer (side surface buffer layer 11 b) may be composed of asilicon oxide film or an aluminum oxide film, etc. These protectivelayers may be formed by a thin film formation method, such as thechemical solution deposition method, sputtering method, MOCVD method,and pulse laser deposition method, separately from formation of thebuffer layer 11 a.

EXAMPLES

Below, the present invention will be explained based on further detailedexamples, but the present invention is not limited to the examples.

Example 1

CaCO₃, Nb₂O₅ and Ga₂O₃ were weighed, fired at 1350° C. in the air, andafter confirming a garnet single phase, put in an iridium melting pot,heated to be about 1450° C. by high frequency induction in a mixed gasatmosphere of 98 volume % of a nitrogen gas and 2 volume % of an oxygengas and melted, so that a composition of the melt becomesCa₃Nb_(1.7)Ga_(3.2)O₁₂. After that, a seed crystal of the abovecomposition having a 5 mm square column shape wherein the long axisdirection is <111> is dipped in this melt to be perpendicular to theliquid surface and pulled up at a speed of 3 mm/hour while rotating at20 rpm, consequently, transparent single crystal having no cracks at allwas obtained.

Next, a sample of about 1 g was cut from each of its upper portion andlower portion of the crystal for quantitative analysis of respectivecomposing elements by X-ray fluorescent analysis apparatus. It wasconfirmed that both of the upper portion and the lower portion of thecrystal had a composition of Ca₃Nb_(1.7)Ga_(3.2)O₁₂ (CNGG).

The obtained single crystal was cut to be a predetermined thicknessvertically to the growing direction, and after performingmirror-polishing on the both sides, etching processing with heatphosphoric acid was performed, so that a CNGG single crystal substrate(base substrate 10) was prepared. The thermal expansion coefficient (a)of the single crystal substrate at the room temperature to 850° C. was1.07×10⁻⁵/° C. The thickness of the CNGG single crystal substrate was0.6 mm.

An Nd₃Ga₅O₁₂ (NGG) thin film (buffer layer 11) was formed by thesputtering method on the bottom and side surfaces of the CNGG singlecrystal substrate. Specifically, an NGG sintered body was used as atarget, sputtering film formation was performed under film formationcondition below, and anneal processing was performed after that.

[Sputtering Film Formation Condition]

substrate temperature: 600° C.

input: 300 W

atmosphere: Ar+O₂ (10 volume %), 1.5 Pa

film formation time: 30 minutes

film thickness of bottom surface buffer layer 11 a: 250 nm

film thickness of side surface buffer layer (side surface protectivefilm) 11 b: 20 nm

[Anneal Processing]

atmosphere: O₂, 1 atm

temperature: 800° C.

time: 30 minutes

A SEM image of the NGG film surface is shown in FIG. 4A. Also, a SEMimage of the section is shown in FIG. 4B. It was confirmed that a flatand smooth NGG film could be obtained. Also, when conducting compositionanalysis of the NGG film by X-ray fluorescent analysis, it was confirmedthat a Nd₃Ga₅O₁₂ (NGG) thin film having an approximate stoichiometriccomposition was obtained.

By using the thus obtained CNGG substrate (substrate 2) added with theside surface and bottom surface NGG films, a bismuth-substituted rareearth iron garnet single crystal film was formed by the liquid phaseepitaxial growth method. Specifically, 5.747 g of Ho₂O₃, 6.724 g ofGd₂O₃, 43.21 g of B₂O₃, 126.84 g of Fe₂O₃, 989.6 g of PbO and 826.4 g ofBi₂O₃ were put in a melting pot made by platinum, melted at about 1000°C. and mixed to be homogenized, then, cooled at a rate of 120° C./hr.and kept in a supersaturated state at 832° C. Next, the substrate 2,wherein the buffer layer is formed also on its side surfaces, obtainedby the above method was dipped in this melt solution and liquid phaseepitaxial growth was performed for 40 hours to grow a single crystalfilm while rotating the substrate at 100 rpm, so that abismuth-substituted rare earth iron garnet single crystal film 12 ahaving a film thickness of 450 μm was formed on the bottom surface ofthe substrate 2. Note that the garnet film 12 b was confirmed to growalso on the side surfaces of the substrate 2.

When analyzing a composition of the single crystal film 12 a formed onthe bottom surface of the substrate 2 by X-ray fluorescent analysismethod, it was confirmed to be Bi_(1.1)Gd_(1.1)Ho_(0.8)Fe_(5.0)O₁₂(Bi-RIG). A SEM image of a section of the single crystal film 12 a isshown in FIG. 5. Also, a result of taking an optical microscope pictureof a surface of the single crystal film 12 a is shown in FIG. 3A.

It was confirmed from the results that a Bi-RIG single crystal filmhaving a flat, smooth and fine surface and of an approximatestoichiometric composition could be formed by epitaxial growth withoutcausing any cracks and flaking. Also, from the picture shown in FIG. 3A,when examining surface defect (etch pit/black points shown in FIG. 3)density by the area ratio, it was 0.04% and it was confirmed that thedefects were a little.

Also, when measuring a difference of a lattice constant of the singlecrystal film and that of the CNGG substrate as a base substrate, it wasconfirmed to be 0.009 Å and within ±0.02 Å. Note that when measuring adifference of the lattice constant of the single crystal film and thatof the NGG thin film as a buffer layer, it was 0.007 Å. Measurement ofthe lattice constant was made by an X-ray diffraction method.

Also, by removing the substrate 2 (the base substrate 10 and the bufferlayer 11) and the garnet film 12 b on the side surfaces from the singlecrystal film 12 a by polishing processing, performing polishingprocessing on the both sides of the single crystal film 12 a, adhering anon-reflection film made by SiO₂ or Ta₂O₅ on both sides thereof, andevaluating a Faraday rotation angle at a wavelength of 1.55 μm and atransmission loss at a Faraday rotation angle of 45 degrees and atemperature characteristic, the Faraday rotation coefficient was 0.125deg/μm, the transmission loss was 0.05 dB, and the temperaturecharacteristic was −0.065 deg/° C. These were all at satisfactory levelsas optical characteristics of an optical isolator.

Note that the Faraday rotation angle was obtained by letting a polarizedlaser light having a wavelength of 1.55 μm enter the single crystal filmand measuring an angle of the deflecting surface of the emitted light.The transmission loss was obtained from a difference of intensity of thelaser light having a wavelength of 1.55 μm passed through the singlecrystal film and light intensity in a state without a single crystalfilm. The temperature characteristic was calculated from a valueobtained by measuring a rotation angle by changing a temperature of thesample from −40° C. to 85° C.

Furthermore, the thermal expansion coefficient (a) of the single crystalfilm at the room temperature to 850° C. was 1.10×10⁻⁵/° C. Thedifference of the thermal expansion coefficient between the basesubstrate and the single crystal film was 0.03×10⁻⁵/° C. Also, arisingof cracks was not observed in the obtained single crystal film.

Comparative Example 1

Other than performing under the normal sputtering condition of notforming an NGG thin film on the side surfaces of the base substrate 10and forming only on the bottom surface thereof when forming the NGG thinfilm as the buffer layer 11 on the base substrate 10 by sputtering, aCNGG single crystal substrate added with an NGG thin film was preparedin the same method as in the above example 1. By using the CNGG singlecrystal substrate added with the NGG thin film, a bismuth-substitutedrare earth iron garnet single crystal film was formed by the liquidphase epitaxial growth method in the same way as the example 1.

A result of taking a picture of a surface of the single crystal film byan optical microscope is shown in FIG. 3B. From the result shown in FIG.3B, when examining surface defect (etch pit/black points shown in FIG.3) density by the area ratio, it was 0.92%. It was confirmed from theresult shown in FIG. 3A that the defects were much.

Comparative Example 2

A CNGG single crystal substrate was prepared in the same method as inthe example 1, and a bismuth-substituted rare earth iron garnet singlecrystal film was formed by the liquid phase epitaxial growth method inthe same way as in the example 1 without forming thereon a buffer layercomposed of a single crystal thin film being stable with a lead oxide.

FIG. 6B is a SEM image of a surface of a substrate after the experiment,and it was confirmed that the surface was etched. Also, it was found bythe fluorescent X-ray analysis that a bismuth-substituted rare earthiron garnet single crystal film was not formed. Note that FIG. 6A is aSEM image of a surface of the single crystal film in the comparativeexample 1 to be compared with the comparative example 2.

Example 2

A CNGG single crystal substrate was prepared in the same method as inthe above example 1.

A Gd_(2.65)Ca_(0.35)Ga_(4.05)Mg_(0.3)Zr_(0.65)O₁₂ (GCGMZG) thin film(buffer layer 11) was formed on the bottom surface and side surfaces ofthe CNGG single crystal substrate (base substrate 10) by the pulse laservapor deposition method. Specifically, a KrF excimer laser wasirradiated at an irradiation laser density of 2.0 J/cm² on a GCGMZGsingle crystal target, and the GCGMZG thin film having a film thicknessof about 10 nm was formed under an oxygen partial pressure of 1 Pa andan irradiation time of 5 minutes on the bottom surface and side surfacesof the CNGG substrate kept at a substrate temperature of 800° C. Whenconducting X-ray fluorescent analysis on the GCGMZG thin film, it wasconfirmed to be a GCGMZG having the same composition as that of thetarget. A film thickness on the side surfaces of the substrate of theGCGMZG thin film was 5 nm.

By using the thus obtained CNGG single crystal substrate added with theGCGMZG thin film, a bismuth-substituted rare earth iron garnet singlecrystal film was formed by the liquid phase epitaxial growth method inthe same way as in the example 1. Arising of cracks was not observed inthe obtained single crystal film.

Example 3

A CNGG single crystal substrate added with an NGG thin film was preparedin the same method as in the above example 1. By using the CNGG singlecrystal substrate added with an NGG thin film, a bismuth-substitutedrare earth iron garnet single crystal film was formed by the liquidphase epitaxial growth method.

Specifically, 12.431 g of Tb₄O₇, 1.464 g of Yb₂O₃, 43.21 g of B₂O₃,121.56 g of Fe₂O₃, 989.6 g of PbO and 826.4 g of Bi₂O₃ are put in amelting pot made by platinum, melted at about 1000° C., mixed to behomogenized, cooled at a rate of 120° C./hr. and kept in asupersaturated state at 840° C. Next, a single crystal substratematerial obtained by forming an NGG thin film of 250 nm on the bottomand side surfaces of a CNGG substrate having a substrate thickness of0.6 mm is dipped in this solution, liquid phase epitaxial growth wasperformed for 43 hours to grow a single crystal film while rotating thesubstrate at 100 rpm, so that a bismuth-substituted rare earth irongarnet single crystal film having a film thickness of 560 μm was formedon the bottom surface of the substrate. Note that thebismuth-substituted rare earth iron garnet single crystal film wasformed also on the side surfaces of the substrate.

Arising of cracks was not observed in both of the obtained singlecrystal film and single crystal substrate. When analyzing a compositionof the single crystal film by the X-ray fluorescent analysis method, itwas confirmed to be Bi_(1.0)Tb_(1.9)Yb_(0.1)Fe_(5.0)O₁₂.

Also, when measuring a difference of a lattice constant of the singlecrystal film and that of the CNGG substrate as a base substrate, it wasconfirmed to be 0.005 Å and within ±0.02 Å. Note that when measuring adifference of the lattice constant of the single crystal film and thatof the NGG thin film as a buffer layer, it was 0.004 Å.

Also, when evaluating a Faraday rotation angle at a wavelength of 1.55μm and a transmission loss at a Faraday rotation angle of 45 degrees andtemperature characteristic, the Faraday rotation coefficient was 0.090deg/μm, the transmission loss was 0.15 dB, and the temperaturecharacteristic was −0.045 deg/° C. Furthermore, the thermal expansioncoefficient of the single crystal film was 1.09×10⁻⁵/° C. The differenceof the thermal expansion coefficient between the base substrate and thesingle crystal film was 0.02×10⁻⁵/° C. Also, arising of cracks was notobserved in the obtained single crystal film.

Example 4

A CNGG single crystal substrate added with an NGG thin film was preparedin the same method as in the above example 1. By using the CNGG singlecrystal substrate added with the NGG thin film, a bismuth-substitutedrare earth iron garnet single crystal film was formed by the liquidphase epitaxial growth method.

Specifically, 7.653 g of Gd₂O₃, 6.778 g of Yb₂O₃, 43.21 g of B₂O₃, 113.2g of Fe₂O₃, 19.02 g of Ga₂O₃, 3.35 g of Al₂O₃, 869.7 g of PbO and 946.3g of Bi₂O₃ are put in a melting pot made by platinum, melted at about1000° C., mixed to be homogenized, cooled at a rate of 120° C./hr. andkept in a supersaturated state at 829° C. Next, a single crystalsubstrate material obtained by forming 250 nm of an NGG thin film on abottom surface and side surfaces of a CNGG substrate having a substratethickness of 0.6 mm is dipped in this solution, liquid phase epitaxialgrowth was performed for 43 hours to grow a single crystal film whilerotating the substrate at 100 rpm, so that a bismuth-substituted rareearth iron garnet single crystal film having a film thickness of 520 μmwas formed on the bottom surface of the substrate. Note that thebismuth-substituted rare earth iron garnet single crystal film wasformed also on the side surfaces of the substrate.

Arising of cracks was not observed in both of the obtained singlecrystal film and single crystal substrate. When analyzing a compositionof the single crystal film by the X-ray fluorescent analysis method, itwas confirmed to be Bi_(1.3)Gd_(1.2)Yb_(0.5)Fe_(4.2)Ga_(0.6)Al_(0.2)O₁₂.

Also, when measuring a difference of a lattice constant of the singlecrystal film and that of the CNGG substrate as a base substrate, it wasconfirmed to be 0.014 Å and within ±0.02 Å. Note that when measuring thedifference of the lattice constant of the single crystal film and thatof the NGG thin film as a buffer layer, it was 0.013 Å.

Also, when evaluating a Faraday rotation angle at a wavelength of 1.55μm and a transmission loss at a Faraday rotation angle of 45 degrees andtemperature characteristic, the Faraday rotation coefficient was 0.113deg/μm, the transmission loss was 0.05 dB, and the temperaturecharacteristic was −0.095 deg/° C. Furthermore, the thermal expansioncoefficient of the single crystal film was 1.05×10⁻⁵/° C. The differenceof the thermal expansion coefficient between the base substrate and thesingle crystal film was 0.02×10⁻⁵/° C. Also, arising of cracks was notobserved in the obtained single crystal film.

EVALUATION

According to the examples 1 to 4, the single crystal film was grownevenly and the crystal surface was smooth and shiny, while according tothe comparative example 1, it was observed that the single crystal filmdid not grow evenly and flaking arose partially as a result thatreaction was caused on a boundary surface of the growing film and thesubstrate.

Also, according to the examples 1 to 4, as shown in FIG. 3, it wasconfirmed that defect density on the surface of the single crystal filmcould be reduced comparing with the comparative example 1.

The above explained embodiments and examples are for illustrating thepresent invention and not to limit the present invention, and thepresent invention can be carried out in a variety of other modifiedembodiments.

EFFECT OF THE INVENTION

As explained above, according to the present invention, it is possibleto provide a magnetic garnet single crystal film formation substratecapable of stably forming a thick magnetic garnet single crystal film,wherein crystal defects, warps, cracks and flaking, etc. are not caused,having a high quality at a high yield by liquid phase epitaxial growth,an optical element and the production method.

1. A magnetic garnet single crystal film formation substrate for growinga magnetic garnet single crystal film by liquid phase epitaxial growth,comprising: a base substrate composed of a garnet-based single crystalbeing unstable with a flux used for the liquid phase epitaxial growth; abuffer layer composed of a garnet-based single crystal thin film formedon a crystal growing surface of said base substrate and being stablewith said flux; and a protective layer formed at least on side surfacesof said base substrate crossing with said crystal growing surface ofsaid base substrate and being stable with said flux.
 2. The magneticgarnet single crystal film formation substrate as set forth in claim 1,including a lead oxide and/or a bismuth oxide as a main component ofsaid flux.
 3. The magnetic garnet single crystal film formationsubstrate as set forth in claim 1, wherein said base substrate has anapproximately same thermal expansion coefficient as that of saidmagnetic garnet single crystal film.
 4. The magnetic garnet singlecrystal film formation substrate as set forth in claim 3, wherein adifference between the thermal expansion coefficient of said basesubstrate and the thermal expansion coefficient of said magnetic garnetsingle crystal film is within a range of ±2×10⁻⁶/° C. or less in atemperature range of 0° C. to 1000° C.
 5. The magnetic garnet singlecrystal film formation substrate as set forth in claim 1, wherein saidbase substrate has an approximately same lattice constant as that ofsaid magnetic garnet single crystal film.
 6. The magnetic garnet singlecrystal film formation substrate as set forth in claim 5, wherein adifference between the lattice constant of said base substrate and thelattice constant of said magnetic garnet single crystal film is within arange of ±0.02 Å or less.
 7. The magnetic garnet single crystal filmformation substrate as set forth in claim 1, wherein said base substrateincludes Nb or Ta.
 8. The magnetic garnet single crystal film formationsubstrate as set forth in claim 1, wherein said buffer layer is agarnet-based single crystal thin film substantially not including Nb andTa.
 9. The magnetic garnet single crystal film formation substrate asset forth in claim 1, wherein said buffer layer is expressed by ageneral formula R₃M₅O₁₂ (note that R is at least one of rare earthelements and M is one selected from Ga and Fe) or an X-substitutedgadolinium gallium garnet (note that X is at least one of Ca, Mg andZr).
 10. The magnetic garnet single crystal film formation substrate asset forth in claim 1, wherein a thickness of said buffer layer is 1 to10000 nm and a thickness of said base substrate is 0.1 to 5 mm.
 11. Themagnetic garnet single crystal film formation substrate as set forth inclaim 1, wherein said protective layer is composed of the same film assaid buffer layer.
 12. The magnetic garnet single crystal film formationsubstrate as set forth in claim 1, wherein said protective layer iscomposed of a different film from said buffer layer.
 13. The magneticgarnet single crystal film formation substrate as set forth in claim 1,wherein said protective layer is composed of a silicon oxide film or analuminum oxide film.
 14. A method of producing a magnetic garnet singlecrystal film, comprising the step of growing a magnetic garnet singlecrystal film on said buffer layer by using the magnetic garnet singlecrystal film formation substrate as set forth in claim 1 by a liquidphase epitaxial growth method.
 15. A method of producing an opticalelement, comprising the steps of forming a magnetic garnet singlecrystal film by using the method of producing a magnetic garnet singlecrystal film as set forth in claim 14, and after that, removing saidbase substrate and buffer layer so as to form an optical elementcomposed of a magnetic garnet single crystal film.
 16. The method ofproducing an optical element as set forth in claim 15, comprising thesteps of: removing said base substrate and buffer layer and removing amagnetic garnet film formed on side surfaces of said base substrate;leaving only a magnetic garnet single crystal film formed on a crystalgrowing surface of said base substrate; and forming an optical elementcomposed of said magnetic garnet single crystal film.
 17. An opticalelement obtained by the method of producing an optical element as setforth in claim 15.